A S A LY TICAL EDI T I O S
232
oxidized by sulfur dioxide in strongly acid solutions. In order to understand or interpret this phenomenon correctly, the oxidation reduction potentials of the ferric-ferrous electrode should be determined in varying acid concentrations. It was found that these potentials decrease with increase of hydrochloric acid concentration being - 0.739 volt in 0.05 M and -0.700 volt in 1.0 1M acid (14). This decrease of potential in the case of the ferric-ferrous electrode, together with the increase of the potential of the sulfurous acid-sulfur electrode, is sufficient to account for the seemingly contradictory results stated formerly in varying acid concentrations. The necessity of dealing only with completely and easily reversible reactions in galvanic cells was emphasized earlier in this paper. The ferric-ferrous electrode is a reversible one and the measured potential is given by: E =Eo-O05915log-
a
Fe++-
aFe+-
* E,
This equation can be reduced to
(MnO4)-
Vol. 2, K O . 3
+ 8H++ 5 ( - )
*&In++
+ 4H2O
This is an unfort'unat,e situation because of the importance of the reaction in many analytical methods. One may venture a calculation employing the potent'ials of the manganese dioxide, hydrogen, and manganous ion electrode, and that of the permanganate, hydrogen ion, and manganese dioxide electrode. Employing the best reported values of these electrodes. the calculation gives -1.48 volts. Brown (3) has obtained a value of -1.446 volts. The question may justly be raised-are not the foregoing electrodes, inyolving manganese dioxide, irreversible ones, especially since they involve a change in the oxygen content? Experiments conducted by the author and his students ( 1 7 ) , in which the utmost care was employed in the analyses of all the constituents entering into the reaction, hai-e shown that any measurements of these electrodes involving the use of manganese dioxide are to be considered as unreliable. Since many of the analytical methods involve reactions in acid solutions, the electrode potentials in varying acid concentrations are of greater importance than the theoretical values for Ed,, the so-called standard electrode potentials (7'). Literature Cited
One can keep the concentration of ferric equal to that of the ferrous iron, and by keeping these concentrations small, of the order of 0.001 the liquid junction potential can be made almost negligible if one employs the hydrogen electrode and the acid concentrations are the same in both half-cells. The decrease of the potential with increase of acid concentration is explained on the basis of the decrease in the ratio of the activity coefficients of the ferric t o ferrous ion. One cannot and should not consider that the ratio of the actirity coefficients n-ould be equal to one, as Linhart (12) and others have done in their studies involving the ferri- and ferrocyanide ions. I t is significant to note that one of the most important electrodes involving the reaction between permanganate, hydrogen, and manganous ions cannot be measured directly because of the irreversibility of the reaction:
(1, Britton, J . Chem. S O L .127, , 2157 (1925). ( 2 ) G r o a n , J . Am. Chem. SOC.,38, 1660 (1916); 48, 582 (1926).
(3) Brown, Unpublished article. Critical Tables, Vol. VI, p . 333 (1929). ( 5 ) Droves and Tartor, J . Am. Chem. SOC.,47, 1226 c192.5). (6) F u r m a n , IND.ENG. C H E X , 15, 62 (1923). ( 7 ) Gerke, International Critical Tables, Val. V I , p . 332 (1929). ( 8 : Hughes, J . Chem. SOC.,1928, 491. (9) Lewis, J . .Am. Chem. S o c . , 28, 158 (1906). (101 Lewis and Randall, I b i d . , 36, 1969 (1914). (11) Linhart, I b i d . , 3 8 , 2356 (1916). (12) LIacInnes and Dole, ISD. EKG.CHEM., Anal. Ed., 1, 57 (1929): J . Gen. Phystol., 12, 805 (1929). (13) Xernst, Z. Eleklrochem., 12, 693 (1906). (14) Popoff and Kunz, J . Phys. Chem., 32, 1056 (1928'1. (15) Popoff, Kunz, and Snow, J . .Am. Chem. Soc., 51, 382 (1929). (16) Popoff and RIcHenry, IND.EX. CHEY., 20, 5 3 1 (1928). (17) Popoff, Riddick, and Becker, J . Am. Chem. Soc., in press. (18) Sand, J . Chem. Sac.. 91,379 ( 1 9 O i ) . (4) Conant, International
[ESDOF SYMPOSIUM]
Improved Apparatus for Obtaining Constant Liquid Flow' William S. Gilfoil LOUISIASAOIL REFIKINGC O R P O R A T I OSHREVEPORT, X, LA.
HIS apparatus has the following advantages over that described b y Sullivan ( 1 ) : It obviates the use of any material except glass, although the use of rubber will simplify its construction. The point of exit of the liquid remains constant. The apparatus consists of three beakers, one of which remains a t a constant level, by continually overflowing into another through an exaggerated lip. The liquid is kept inoviiig by siphons and an air lift, a stopcock, 1'. regulating the flow. Another siphon, n i t h a stopcock, gives constant flow at the fixed point, A . Literature Cited (1)
Sullivan, IND ENG C H E WAnal , Ed 1
Received May 3, 1930.
, 1, 233 (1929).
